Add optional markitdown extraction for Office/EPUB documents (#766)

Office documents were dropped server-side: .docx fell through to
"[Attached document file]", .xlsx/.pptx weren't recognized at all, and
the personal-docs RAG index only covered txt/md/json/pdf.

Wire the optional markitdown dependency (MIT, Microsoft) into both the
chat-attachment path (build_user_content) and the RAG indexer
(personal_docs), converting .docx/.xlsx/.pptx/.xls/.epub to Markdown.
It is lazy-imported with graceful fallback (mirrors src/pdf_runtime.py):
without it those formats show an "install to extract" banner and the
MIT core is unaffected. pypdf stays the default PDF path.

- src/markitdown_runtime.py: optional-dep loader + convert_to_markdown
- upload_handler: recognize Office/EPUB extensions + MIME types
- document_processor: extract Office docs in the chat else-branch
- personal_docs: index Office docs (DEFAULT_EXTENSIONS + dispatch)
- requirements-optional.txt + ACKNOWLEDGMENTS.md: pinned markitdown 0.1.5
- tests: markitdown_runtime + office index coverage

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/upload_handler.py

@@ -128,7 +128,8 @@ class UploadHandler:
     def is_document_file(self, filename: str, content_type: str = None) -> bool:
         """Check if a file is a document based on extension or content type."""
         document_extensions = {
-            '.pdf', '.docx', '.txt', '.py', '.js', '.html', '.htm', 
+            '.pdf', '.docx', '.xlsx', '.pptx', '.xls', '.epub',
+            '.txt', '.py', '.js', '.html', '.htm',
             '.css', '.json', '.md', '.csv', '.log', '.xml', '.yml', 
             '.yaml', '.sql', '.sh', '.bash', '.c', '.cpp', '.h', 
             '.java', '.go', '.rs', '.php', '.rb', '.ts', '.jsx', '.tsx'
@@ -136,6 +137,10 @@ class UploadHandler:
         document_mime_types = {
             'application/pdf', 
             'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
+            'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
+            'application/vnd.openxmlformats-officedocument.presentationml.presentation',
+            'application/vnd.ms-excel',
+            'application/epub+zip',
             'text/plain'
         }